1. Field of the Invention
This invention relates to a projection printing apparatus for printing a photomask on a wafer, by the use of a projection lens, in the manufacture of integrated circuits.
2. Description of the Prior Art
A method of printing a photomask on a wafer in the manufacture of integrated circuits is known in which printing is carried out with the mask and the wafer superposed one upon the other, with a printing projection system disposed between the mask and the wafer so that projection exposure is effected thereby, and in which the mask and the wafer are scanned on an arcuate slit-like picture plane area by the use of a mirror optical system to thereby effect exposure. Particularly, recently, it is increasingly necessary to print minute circuit patterns and for that reason, attention has been paid to projection exposure systems using a lens having a high resolving power. However, a lens having such a high resolving power and capable of printing the entire surface of a wafer having dimensions of four or five inches at one time is difficult to designs and manufacture. Accordingly, where a wafer of large diameter is to be printed by the use of this system, there has been proposed a method whereby the exposure area EA on the wafer is divided into several rectangular areas RA is shown, for example, in FIG. 1 of the accompanying drawings, and these rectangular areas are successively exposed to light to thereby complete the exposure of the entire wafer surface.
Now, when a photomask is to be thus printed on a wafer in the integrated circuit manufacturing process, it is necessary to observe a pattern already formed on the wafer in the previous step of the process and a portion of the photomask to be next printed, at the same time, and to adjust the position of the photomask or the wafer until they assume a predetermined positional relation, namely, to effect alignment. At that time, the wafer is observed through a projection lens and the illumination light therefore must be a light of a such a wavelength to which the photoresist applied onto the wafer is not sensitized. Also, where a mechanism for automatically effecting alignment is used, it becomes necessary to cause a light of a different wavelength such as laser light to be imaged through the projection lens. However, the printing projection lens so used for the manufacture of integrated circuits is required to be highly aberration-corrected in order to print minute patterns. However, it is difficult to correct the aberrations for light of numerous wavelengths by the use of presently available optical materials.
For this reason, the applicant proposed, in U.S. Pat. No. 3,897,138, a projection printing method whereby different imaging optical systems are inserted between the mask and the wafer during printing and during alignment.
This method, however, has suffered from a problem in adjustment of the optical axis which results from the interchange of the imaging optical systems.